1. Field of the Invention
The present invention is generally directed toward pressure relief devices. In particular, the present invention relates to pressure relief devices that include a line of opening formed by selective operation of a laser to create a plurality of co-linear recesses interspersed with a plurality of gap segments.
2. Description of the Prior Art
Score lines have been for some time in rupture discs to define an area of the disc that opens upon exposure to a predetermined overpressure condition. During opening, the rupture disc tears at the score line to create one or more petals that pivot about one or more respective hinge regions under the force of the overpressure condition to permit a pressurized fluid to flow through the opened disc.
Score lines are typically created through the use of metal scoring dies. When using a metal scoring die to form the score line, the die compresses and work hardens the disc metal, thus changing the grain structure of the metal. This work hardening can increase the brittleness of the metal and create stress zones. The brittleness and stress zones limit the service life of the pressure relief device as a result of fatigue cracking and stress corrosion. Additionally, it is difficult to achieve a high degree of control over the depth of the score in disc manufacturing, given that the die itself becomes worn during the scoring process and must be periodically replaced. This lack of control imparts a degree of unpredictability into the disc opening characteristics.
In order to avoid the issues with die scoring, alternate processes have been devised to form lines about which the rupture disc opens. One such process is described in U.S. Pat. No. 7,600,527 discloses forming a line of weakness through an electropolishing method. In this process, a rupture disc is provided with a layer of resist material. A laser is then used to remove a portion of the resist material corresponding to the desired line of weakness. Next, the disc undergoes an electropolishing operation to remove metal from the surface of the disc thereby forming a line of weakness having a desired depth. However, controlling the width of the electropolished line of weakness can be difficult, particularly if the disc material is relatively thick and requires extended electropolishing times in order to achieve the desired line depth.
It has also been suggested to directly use a laser to machine a line of weakness into a rupture disc. U.S. Patent Application Publications 2010/0140264 and 2010/0224603 are exemplary in this regard. However, these references adopt conventional line of weakness configurations insofar as the lines of weakness comprise relatively long, continuous recesses formed in one face of the disc which can limit the range of burst pressures than can be created for a disc of a particular thickness.